Field of the Disclosure
The present disclosure relates to spectroscopic instruments and methods, and more specifically, to an optical instrument for biomechanical diagnosis of eye disease.
Description of the Related Art
Various diagnostic and clinical instruments have been developed for in vivo imaging of biological issues, and in particular, for imaging structures in a human eye. Specifically, optical instruments are used to measure geometrical and optical characteristics of different sections of the human eye. The geometric and optical characteristics provided by such analyses may enable various degrees of biomechanical or physiological modelling of an individual eye of a patient in an effort to diagnose an ophthalmological health condition and to develop a suitable treatment plan.
Keratoconus is a degenerative disease of the human eye, which is characterized by a noninflammatory thinning and steepening of the curvature in the central or paracentral cornea, resulting in a conical cornea that characterizes the disease. The structural changes in the cornea as keratoconus progresses cause significant impairment of vision in a patient. Structural changes of the cornea caused by keratoconus may further complicate certain laser vision correction surgeries, such as laser assisted in situ keratomileusis (LASIK) surgery or photorefractive keratectomy (PRK) surgery, because of possible increased risks from post-surgical corneal ectasia (or thinning).
The visual impairment caused by keratoconus may be corrected to a degree using specially adapted eyeglasses or corneoscleral contact lenses. Such measures, however, may not be effective when keratoconus advances to a late stage of pathogenesis. For advanced stage keratoconus, a corneal crosslinking treatment is performed, which may stop or at least decelerate pathogenesis, although complete visual rehabilitation is not presently prognosticated.
Besides keratoconus, other types of corneal degeneration may impact the biomechanical stability of the human cornea. For example, pellucid marginal corneal degeneration (PMD), which is also known as keratotorus, is a degenerative noninflammatory corneal condition, which is typically characterized by a clear, bilateral ectasia in the inferior and peripheral region of the cornea. In particular, patients with PMD may exhibit normal thickness in the center of the cornea with an intact central epithelium, while exhibiting a peripheral band of thinning in the inferior cornea. The portion of the cornea that is immediately adjacent to the limbus may be spared by PMD, and typically includes a strip of about a few millimeters. As a further result of PMD, Bowman's layer of the cornea may be absent, irregular, or may have ruptured areas.
Optical examinations of the viscoelastic properties of eye tissues, such as the cornea and the eye lens, have been performed using Brillouin scattering (BS), which relies on position-dependent mass density variations inside a sampled material to scatter incident coherent light by means of a phonon-photon interaction. By spectroscopically analyzing a Brillouin scattered light beam from eye tissue, certain biomechanical properties of the eye tissue may be determined, such as a viscoelastic property of the analyzed eye tissue. The determination of the viscoelastic properties of eye tissue using Brillouin scattering may be particularly useful in connection with keratoconus crosslinking treatments of the cornea using ultraviolet (UV) light, in order to probe and document an actual qualitative impact of the crosslinking treatment on relevant biomechanical properties of the cornea, such as hardness and stiffness.
Furthermore, the fine biostructure of the cornea may be examined using multiphoton microscopy to generate certain images of eye tissue. During multiphoton imaging, different structures in the cornea are excited through multiphoton absorption and may undergo autofluorescence. Alternatively, certain non-isotropic structures, such as collagen fibrils in the cornea, may used for second harmonic generation (SHG) or third harmonic generation (THG) of wavelengths of a sample beam through non-linear interaction. In this manner, SHG or THG signals may indicate the position and distribution of various structures in the cornea.
In particular, SHG signals have been associated with the position and distribution of the fibrils in the cornea. The arrangement and orientation of corneal fibrils, as well as their position and density, may correlate with the optical and the mechanical properties of the cornea in the regions analyzed using SHG signals. The optical properties may include transparency and scattering, while the mechanical properties may include elastomechanical properties, such as strength. It is known that in advanced stages of eye diseases, such as keratoconus and corneal dystrophy, changes in the optical and mechanical properties of affected eye tissue are detectable in the coarse structure of the cornea. Therefore, analysis using SHG signals may be a suitable method to examine the fine structure of the cornea and may enable early detection of the pathogenesis of eye disease. Additionally, SHG signals are generated in vivo from corneal fibrils during certain LASIK surgeries with femtosecond (fs) lasers and may provide insight into directionality and position of LASIK incisions.